1. Field of the Invention
This invention relates to a novel method for removing undesirable organic substances present in low concentrations from a gas stream by adsorbing the contaminants onto a sorptive media and subsequently destroying the contaminants by catalytic oxidation and/or hydrolysis, referred to herein simply as oxidation.
More particularly, this invention provides a practical route for eliminating even trace amounts of contaminants from an oxygen containing inert gas stream by passing the stream through an adsorptive bed which is in intimate contact with an oxidation catalyst, followed by thermal desorption of the contaminant and its subsequent conversion to innocuous products. Specifically, it may be used to effectively remove vinyl chloride monomer, chloroprene and vinylidene chloride from contaminated air streams.
The removal of vinyl chloride from the atmosphere has particular relevance at this time because it is believed hazardous to health and is associated with tumor-inducing activity. As a result, considerable effort has been directed to eliminating or at least reducing the concentration of vinyl chloride in work areas and its environs.
2. Description of the Prior Art
There are several known methods for removing or reducing the concentration of dilute contaminants in the atmosphere. In general, these methods fall into two broad categories: (1) contacting the contaminated gas with a liquid solvent which selectively absorbs the contaminant, or (2) adsorbing the contaminant onto a solid adsorbent.
The first of the aforementioned processes, namely, the absorption of a contaminant via the use of a solvent, is illustrated by Bellisio in U.S. Pat. No. 3,807,138. The principle disadvantage in the Bellisio method is its inability to effectively remove trace quantities of vinyl chloride monomer. Furthermore, the process requires that the feed gas be compressed to an elevated pressure for efficient dissolution of vinyl chloride monomer making it economically unattractive when large volumes of feed has to be handled. Other disadvantages are high cost of solvent and loss of solvent during the stripping step.
The second principal method heretofore employed for the removal of contaminants in a gas is illustrated by Raduly, U.S. Pat. No. 3,796,023. That patent describes a method for removing vinyl chloride monomer by passing inert gases containing the contaminant over a single layer of activated carbon maintained at a temperature of -10.degree. to 35.degree. C. followed by the desorption of the vinyl chloride by heating the layer with steam and the separation of the desorbed vinyl chloride monomer from the steam by condensation of the water vapor. The main disadvantage to this procedure is that upon contacting the carbon bed with live steam the adsorptive capacity of the activated carbon for vinyl chloride monomer is either destroyed or substantially lessened. Therefore, regeneration of the activated carbon requires the removal of adsorbed and void water and such regeneration necessitates an additional stream of inert, hot gas. Furthermore, when the monomer content of the feed stream is very low the cost of recovery by this method is uneconomical.
A variation on the solid-adsorption process is illustrated by the two-stage method of French Pat. No. 2,194,469. The first stage acts as an adsorber which removes the contaminants from the gas stream. The adsorbed molecules are then desorbed by (1) passing a cleaning gas through the first stage or, (2) by applying a vacuum to the first stage and passing the desorbed contaminants through a second stage consisting of a packed column containing an oxidation catalyst. In this second stage, the contaminants are oxidized with the application of heat to non-toxic products.
The two-stage method of French Pat. No. 2,194,469 appears to be an improvement over known adsorption processes, but it has several disadvantages. First, it is essential that the contaminants adsorbed in the first stage be removed as completely as possible from the adsorptive support. This is effected by either of two methods: (1) by passing a clean stream of gas through the first stage so as to elute the adsorbed contaminants, or (2) by evacuating the first stage following the adsorption cycle. Either method is impractical: (1) elution requires large quantities of a cleaning gas for complete desorption of the adsorbed impurities and (2) evacuation procedures require a low pressure level in order to clean the column efficiently. The feasibility of evacuation depends upon the affinity of adsorption; the stronger the affinity of the adsorbent material for the contaminant, the lower the efficient desorption pressure; hence, this approach necessitates substantial energy expenditures.
In addition, the two-stage French process requires that the contaminants obtained from the first stage be passed through the second stage support together with a large quantity of the cleaning gas if desorption from the first stage is carried out by elution. As a practical matter this means that a large volume of gas has to be heated to the reaction temperature in order to achieve the desired oxidation in the second stage. Such a procedure obviously requires a large expenditure of energy and, therefore, it is neither practical nor efficient. Also, it should be noted that if the desorption of the concentrated adsorbed contaminants from the first stage layer is carried out by elution, then the concentration of the contaminant is rediluted and the advantage of concentrating the impurities in the first stage is lost.
Another method for purifying air is set forth in an article entitled "Catalytic Reaction of Activated Carbon in Air Purification Systems" by Amos Turk, Industrial and Engineering Chem., Vol. 47, pages 966 to 971 (1955). The method, which employs a catalyst of a Group VI-A metal oxide such as chromic oxide deposited on activated carbon, comprises saturating the carbon with an organic contaminant in a stream of circulating air, heating an air stream of reduced quantity to oxidation temperatures, e.g., 220.degree. to 350.degree. C., and recirculating the heated air to oxidize the contaminants and to reactivate the carbon adsorbent. The Turk reference teaches that catalyst agents such as platinum and palladium deposited on carbon are not applicable to this method because they would render the carbon pyrophoric at temperatures used to reactive the catalyst. This method suffers from some of the same disadvantages noted above in connection with French process.
The process of this invention overcomes the disadvantages of prior art methods by providing both an efficient means of removal and destruction of oxidizable dilute organic contaminants from air and other inert gas streams containing molecular oxygen.